Stiff Arm

What does Stiff Arm Mean?

A movement made by an offensive player in which he extends his
arm to ward off a defending player attempting to tackle him. A
player can use a stiff arm to break a tackle or gain extra yardage.
A particularly good stiff arm will actually knock over the
defending player, allowing the ball carrier to extend the play
considerably. Stronger players will be more effective at using the
stiff arm.

Sporting Charts explains Stiff Arm

The stiff arm is a classic part of a great running back's
arsenal of tricks. Its goal is to ward off a defender, and it can
be effective even if it does not knock the player over because it
limits the defender's ability to get his hands on the ball carrier
and make an effective tackle. Generally, the longer a player can
keep the defender from wrapping him up, the more time he will have
with his legs free to gain more yards. A highlight reel stiff arm
will knock over the defender, allowing a free run down the field.
Defensive players can counter the stiff arm by either fighting
through it or grabbing the offensive player's arm and trying to
swing him to the ground with it.